The present invention relates to an electric power steering system for a motor vehicle, and more particularly to a system for controlling a motor in accordance with the turning of a steering wheel.
The electric power steering system has a torsion torque sensor for measuring torsion torque in a steering shaft. The system is adapted to produce an assist signal to control the rotational direction and torque of a motor in dependency on a torsion torque signal from the sensor (Japanese Patent Application Publication No. 45-41246). The response of the steering system and the returnability of the steering wheel may deteriorate because of the inertia of the motor, the friction of a reduction gear train provided in the system, and other factors.
There has been proposed a control system which may prevent the above-described disadvantages (U.S. Pat. No. 4,664,211). The system is provided for producing an assist signal dependent on the torsion torque signal from the torsion torque sensor and on the vehicle speed, and a return torque signal depending on an output signal of a steering angle sensor, for returning the steering wheel to a neutral position. The signals are added for controlling the motor.
In this steering system, the return signal is determined only by the steering angle sensor. Accordingly, if the return signal is set to provide a sufficient return torque when the lateral acceleration exerted on the motor vehicle during cornering is in a low range, when the lateral acceleration is high (for example, higher than 0.3 G) during cornering at a low or middle vehicle speed (for example, 20 Km/h to 60 Km/h), the return starting speed of the steering wheel becomes insufficient, resulting in delay of the returning of the steering wheel.